which implies that \(u\equiv 0\) . Point out that for \(\lambda >0\) , we cannot always use the doubling lemma technique as in the case \(\lambda =0\)  (see [1]}), to reduce the classification for only bounded solutions of (REF ) or (REF ). Therefore, we shall pay special attention to the delicate case of unbounded solutions by exploiting the following variant of the monotonicity formula.

